1) Field of the Invention
This invention relates generally to the field of refrigerators and heat pumps/air conditioners; and, more specifically, it discloses a stand-alone window refrigerator that has its condenser coil placed outdoors. Like a window air conditioner (A/C), a window refrigerator's back (its air-cooled condenser) is set outdoors to save electricity whenever the kitchen is warmer than the outdoors (which, because of the oven, the indoor refrigerator, and the dishwasher, is most of the time). In addition, in the winter, cold outdoor air can be used to cool the inside of the refrigerator. During the hot summer months, this device can be used as both a refrigertor/freezer (R/F) and an indoor cooling and de-humidifying device; and in winter as an indoor heating-humidifying device.
A refrigerator may also be built into a through-the-wall heat pump, which is usually installed under the window through an opening in the wall. Some heat pumps have both cooling and heating elements as are often found in hotel rooms, condominiums, and office buildings. Another variation can be a thermos that is attached to the front of the window A/C.
2) Description of the Related Art
A heat pump is a machine which transfers or moves heat from a low temperature reservoir to a higher temperature reservoir under supply of work. Refrigerators, freezers, air conditioners, and some heating systems are all common applications of heat pumps. They all have the same internal components: compressor, condenser, refrigerant, evaporator, pump, motor and fan. In the summer, a heat pump serves as an A/C by absorbing heat from indoor air and pumping it outdoors. In the winter, it does the reverse by absorbing heat from outdoor air and pumping it indoors. An A/C is basically a refrigerator without the insulated box. Refrigerators and A/Cs are both examples of heat pumps operating only in the cooling mode. A refrigerator is essentially an insulated box with a heat pump system connected to it. The evaporator coil is located inside the box, usually in the freezer. Heat is absorbed from this location and transferred outside, usually behind or underneath the unit where the condenser coil is located. Similarly, an A/C transfers heat from inside a house to the outdoors.
The most common heat pump efficiency measurement is called the Coefficient of Performance, or COP. The COP is the ratio of the heat pump's BTU heat output to the BTU electrical input. The higher the COP, the better; because more heat can be transferred using less work (electricity). The COP depends primarily on the temperatures of the evaporator (inside the R/F) and the condenser (on the back of the R/F). The closer the two temperatures, the higher the COP. Therefore, the colder the outside temperature gets, the closer it gets to the inside temperature of the R/F, and the higher the efficiency of the window R/F relative to a similar indoor R/F.
Operation of an A/C at elevated ambient temperatures inherently results in a lower COP. Generally speaking, when cooling, for each 1° C. reduction in air-conditioning temperature, energy consumption goes up about 10%. This conclusion comes directly from examining the Carnot cycle. The COP relation, COP=Tevap/(Tcond−Tevap), indicates that the COP decreases when the condenser temperature increases at a constant evaporation temperature. This theoretical indication derived from the reversible cycle is valid for all refrigerants. For refrigerants operating in the vapor compression cycle, the COP degradation is greater than that for the Carnot cycle and varies among fluids. The two most influential fundamental thermodynamic properties affecting this degradation are a refrigerant's critical temperature and its molar heat capacity (e.g., McLinden, 1987; Domanski, 1999). For a given application, a fluid with a lower critical temperature will tend to have a lower COP.
Conversely, the COP for a heat pump decreases as the outdoor temperature decreases, because it is more difficult to extract heat from cooler air. Conventional electric resistance heaters have a COP of 1.0. This means it takes one watt of electricity to deliver the heat equivalent of one watt. Air-source heat pumps generally have COPs ranging from 2 to 4; they deliver two to four times more energy than they consume. Water and ground source heat pumps normally have COPs of somewhere between 3 and 5.
According to the US Energy Information Administration's website, in 2001, refrigerators consumed 14% of the total amount of electricity in the average US household—the most of all appliances (the separate freezer unit consumed an additional 3%). The refrigerator consumes more electricity than the computer, computer monitor, television, printer, copier, and clothes dryer. It even consumes more electricity per year than the window or room A/C (2%). This is because, unlike the A/C, the refrigerator is a necessity that is never turned off.
When it is 100° F. outdoors, the window A/C consumes much more electricity (its COP is lower) than when it is 80° F. outdoors. The same is true of a window refrigerator. When the outdoor temperature is 60° F., 50° F., 40° F., or 30° F., the window refrigerator consumes far less electricity (its COP is much greater) than an indoor refrigerator that is in a 70° F. or 80° F. kitchen day and night all year. Indoor refrigerators generate noise and heat. The heat warms the indoor space in the hot summer months, adding to the discomfort.
The indoor refrigerator works against the A/C, warming the home and wasting electricity. In the summer, a window refrigerator/freezer (R/F) does not heat the home (work against the A/C) as does an indoor refrigerator. Even when the A/C is on, the kitchen is often warmer than the outdoors. People use the window A/C to cool their living space (not the kitchen). The R/F is placed in the kitchen near the oven and the dishwasher. Because of the oven, the indoor refrigerator and the dishwasher, the kitchen is often the hottest room in the home and the R/F's door is frequently opened during cooking when the oven is hot. All these factors add to the inefficiency of the indoor refrigerator (lower COP relative to a similar window R/F) and increase its electricity consumption.
The latent heat of fusion of water is (from ice to water) 80 calories of heat per gram and the latent heat of vaporization is (from water to vapor) 540 Calories/Gram. That means, at one atmosphere of pressure, water will absorb about 550 calories of heat per gram when changing from water at 100° C. to water vapor at 100° C. (and vice versa). And it will absorb about 80 calories when changing from ice at 0° C. to water at 0° C. (and vice versa). To save electricity, in the summer, the window R/F freezes water at night when the outdoors is cold. During the day when it is hot outdoors, the ice that was frozen the previous night is melted to aid in keeping the refrigerator compartment cool.